Device-to-device (D2D) communication is a well-known and widely used component of many existing wireless technologies, including ad hoc and cellular networks. Examples include Bluetooth and several variants of the IEEE 802.11 standards suite, such as WiFi Direct. These example systems operate in unlicensed spectrum.
Although the idea of enabling D2D communications as a means of relaying in cellular networks was proposed by some early works on ad hoc networks, the concept of allowing local D2D communications to (re)use cellular spectrum resources simultaneously with ongoing cellular traffic is relatively new. Because non-orthogonal resource sharing between the cellular and the D2D layers has the potential of reuse gain and proximity gain, along with increased resource utilization, the concept of D2D communications underlying cellular networks has received considerable interest in recent years.
The Third Generation Partnership Project (3GPP) refers to Network Controlled D2D as “Proximity Services” or “ProSe,” and efforts aimed at integrated D2D functionality into the Long Term Evolution (LTE) specifications are underway. The ProSe Study Item (SI) recommends supporting D2D operation between wireless devices—referred to as user equipments or UEs by 3GPP—that are outside of network coverage, as well as operation between in-coverage and out-of-coverage wireless devices.
Specifically, in 3GPP LTE networks, such LTE Direct (D2D) communication can be used in commercial applications, such as cellular network offloading, proximity-based social networking. D2D communications involving out-of-coverage operation are expected to be particularly important in so-called national security and public safety services (NSPS), such as in public safety situations in which first responders need to communicate with each other and with people in a disaster area. In some of these applications, it may be the case that communication is largely or entirely in one direction, i.e., one-to-many communications, where one device is sending and multiple devices are listening. However, both commercial and public safety applications are among the use cases discussed in the feasibility study performed by members of the 3rd-Generation Partnership Project (3GPP) and documented in the report “3rd Generation Partnership Project; Technical Specification Group Services and System Aspects; Feasibility study for Proximity Services (ProSE),” 3GPP TR 22.803, v. 12.2.0 (June 2013), available at www.3gpp.org.
D2D communication entities using an LTE Direct link may reuse the same physical resource blocks (PRBs), the basic time-frequency resource in the LTE radio link) as used for cellular communications either in the downlink or in the uplink or both. The reuse of radio resources in a controlled fashion can lead to the increase of spectral efficiency at the expense of some increase of the intra-cell interference.
Typically, D2D communicating entities in an LTE-underlying scenario will use uplink (UL) resources, such as UL PRBs or UL time slots, but conceptually it is possible that D2D (LTE Direct) communications take place in the cellular downlink (DL) spectrum or in DL time slots. For ease of presentation, in the present disclosure it is assumed that D2D links use uplink resources, such as uplink PRBs in a Frequency-Division Duplexing (FDD) LTE system, or uplink time slots in an a cellular Time-Division Duplexing (TDD) system, but the ideas disclosed herein may be readily applied to cases in which D2D communications take place in DL spectrum as well.
An important aspect of D2D communications is the concept of “discovery,” which deals with the techniques and procedures used by a D2D device to detect the proximity of other devices that are capable of and authorized to engage in D2D communications.
The 3GPP technical report titled “Study on architecture enhancements to support Proximity-based Services (ProSe),” briefly outlines several approaches to discovery, including an “LTE based solution for direct discovery.” (3GPP TR 23.703, sec. 6.1 v 1.1.0 (January 2014).) According to this approach, an “announcing UE,” i.e., a wireless device that wishes to make its services or need for services known to other devices receives a so-called expression code from a ProSe function in or attached to the LTE network, e.g., in a ProSe server, and broadcasts the received expression code. Other wireless devices, referred to in 3GPP documentation as “monitoring UEs” monitor for broadcasts from announcing UEs and compare any received expression code to one or more expression codes that they may have previously received, to detect the presence of announcing UEs they are interested in communicating with.
In general, an important requirement for any widely used communication protocol is that it can evolve and extend in a backward compatible manner. For this reason, 3GPP radio interface protocol stacks are based on the concept of access stratum (AS) release. The main principle behind the AS release concept is that new functions, features and corrections to the AS specifications are grouped together and introduced in a new release, i.e., a new version, of the protocol stack. In operation, a mobile device indicates its AS version to the network side of the radio interface, e.g., by sending an indicator that indicates whether the mobile device supports Release 8, 9, 10, etc., of the 3GPP specifications applicable to the mobile device. AS version is indicated by using dedicated Radio Resource Control (RRC) protocol signaling. The network side is responsible for configuration and control of the mobile device, and network nodes are able to handle different versions of the protocols. Therefore, network nodes do not have AS release indicators and they do not need to indicate anything about their implemented versions back to the mobile device.